Estrogen receptor β represses Akt signaling in breast cancer cells via downregulation of HER2/HER3 and upregulation of PTEN: implications for tamoxifen sensitivity
1 Department of Biosciences and Nutrition, Novum, Karolinska Institutet, Blickagången 6, S-141 83 Huddinge, Sweden
2 Department of Organic Chemistry and Natural Products, University of Aveiro, Avenida da Universidade, 3810-193 Aveiro, Portugal
3 Center for Nuclear Receptors and Cell Signaling, Department of Biology and Biochemistry, 3605 Cullen Boulevard, SERC Bldg 545, University of Houston, Houston, TX 77204-5056, USA
Breast Cancer Research 2011, 13:R43 doi:10.1186/bcr2865Published: 14 April 2011
The inhibition of estrogen receptor (ER) α action with the ER antagonist tamoxifen is an established treatment in the majority of breast cancers. De novo or acquired resistance to this therapy is common. Expression of ERβ in breast tumors has been implicated as an indicator of tamoxifen sensitivity. The mechanisms behind this observation remain largely uncharacterized. In the present study, we investigated whether ERβ can modulate pathways implicated in endocrine resistance development.
T47-D and MCF-7 ERα-expressing breast cancer cells with tetracycline-regulated expression of ERβ were used as a model system. Expression levels and activity of known regulators of endocrine resistance were analyzed by performing quantitative polymerase chain reaction assays, Western blot analysis and immunostaining, and sensitivity to tamoxifen was investigated by using a cell proliferation kit.
Expression of ERβ in ERα-positive T47-D and MCF-7 human breast cancer cells resulted in a decrease in Akt signaling. The active form of an upstream regulator of Akt, proto-oncogene c-ErbB-2/receptor tyrosine kinase erbB-3 (HER2/HER3) receptor dimer, was also downregulated by ERβ. Furthermore, ERβ increased expression of the important inhibitor of Akt, phosphatase and tensin homologue deleted on chromosome 10 (PTEN). Importantly, ERβ expression increased the sensitivity of these breast cancer cells to tamoxifen.
Our results suggest a link between expression of ERβ and endocrine sensitivity by increasing PTEN levels and decreasing HER2/HER3 signaling, thereby reducing Akt signaling with subsequent effects on proliferation, survival and tamoxifen sensitivity of breast cancer cells. This study supports initiatives to further investigate whether ERβ presence in breast cancer samples is an indicator for endocrine response. Current therapies in ERα-positive breast cancers aim to impair ERα activity with antagonists or by removal of endogenous estrogens with aromatase inhibitors. Data from this study could be taken as indicative for also using ERβ as a target in selected groups of breast cancer.